Bridge plugs are tools which are typically lowered into a cased oil or gas well. When set in position inside the casing, a bridge plug provides a seal to isolate pressure between two zones in the well. Retrievable bridge plugs are often used during workover operations when a temporary separation of zones is required.
Typical bridge plugs are shown in U.S. Pat. No. 4,436,150 issued to Barker on Mar. 13, 1984; U.S. Pat. No. 4,898,239 issued to Rosenthal on Feb. 6, 1990; and U.S. Pat. No. 5,727,632 issued to Richards on Mar. 17, 1998. Retrievable bridge plugs typically have anchor elements and sealing elements. The anchor elements are used to grip the inside surface of the well casing, thereby preventing the bridge plug from moving up or down within the casing, once set. The sealing elements engage the inside surface of the well casing to provide the requisite seal between the plug and the casing. Typically, the bridge plug is set in position by radially extending the anchor elements and the sealing elements to engage the well casing. To retrieve the bridge plug from the well casing, a retrieving tool is lowered down the casing to engage a retrieving latch, which, through a retrieving mechanism, retracts the anchor elements and the sealing elements, allowing the bridge plug to be pulled out of the well bore.
During workover operations, a pressure differential across the plug often develops. It is desirable to equalize this pressure differential before the anchor and sealing elements are disengaged. Equalization prevents the loss of control over the bridge plug, wherein the tool may be blown up or down a well casing in response to the pressure differential. As exemplified by the prior art bridge plugs listed above, such equalization is typically effected through the opening of a bypass passage through the interior of the plug, prior to disengagement of the anchor and sealing elements.
However, a problem is often encountered with the effect of debris on the operation of the plug. Such debris may have an adverse effect on the operation of the plug. If sufficient debris remains on top of the plug, it may block the proper functioning of the mechanism used to open the bypass passage, making it very difficult, if not impossible for the pressure to equalize across the plug. Further, if a significant amount of debris accumulates on top of the bridge plug, it may be difficult, or impossible to engage the retrieving latch to retract the anchor and sealing elements. Finally, debris accumulation inside the bridge plug may adversely affect the relative movement of various parts within the bridge plug.
With prior art retrievable bridge plugs, even with the bypass passage sealed, fluid in the well is allowed to enter the interior of the plug. Further, no structure protects the uphole opening of the bypass passage, and debris is allowed to accumulate adjacent this opening. Moreover, no means are provided by the bridge plug to protect, or clear the retrieving latch used to disengage the anchor and sealing elements.
With known retrieving tools, once the retrieving tool has latched onto the bridge plug, accidental unlatching of the bridge plug may occur due to jarring motions, or forces imparted on the bridge plug or retrieving tool when the bypass passage is opened. Also, while the bridge plug is being lifted out of the well bore, jarring, or friction against the well casing may cause the anchor elements to move to their extended positions, locking the bridge plug in place within the casing, necessitating emergency recovery procedures. Once the retrieving tool and bridge plug have been removed from the well bore, it is difficult to separate the two, additional machinery often being required.